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91755213-Geology：Giant sheath-folded nappe stack demonstrates extreme subhorizontal shear strain in an Archean orogeny
Abstract:Giant sheath-folded nappes are associated with suture zones and emplacement of far-traveled allochthons in Phanerozoic orogens, demonstrating a rare but significant geologic phenomenon indicative of modern-style plate tectonics. We document the world’s oldest-known subhorizontal mega-scale sheath fold from Archean Alpine-style nappes of the Central orogenic belt, North China craton. The Zanhuang nappes are recumbent Alpine-style forearc-affinity metabasaltic and metasedimentary nappes emplaced over a passive continental margin in the Archean, marking an ancient suture zone. Field evidence shows multiscale sheath folds from decimeters to tens of meters in size, and our three-dimensional fence profile, fold hinges, kinematic lineations, and lithological traces define an ~1-km-long (parallel to the x-axis) sheath fold in the core of the nappe stack. Structural analysis statistically demonstrates the macro-scale recumbent sheath-folded nappe preserves a complete 180° hinge-line curvature. The giant sheath fold plunges northwest, reflecting its formation during non-coaxial, top-to-the-southeast shearing with extremely high shear strain (γ ≥10), equated to >10 km of ductile slip on the bounding surfaces. Slip vectors derived from S-C fabrics on overturned limbs are consistent with rotation into the southeast-directed transport direction, parallel to the similarly rotated fold hinges. Comparison of the giant sheath-folded nappes from the Archean Zanhuang example with mega-scale sheath folds in Phanerozoic and Proterozoic orogens shows that Neoarchean lithosphere was stiff enough to allow tectonics to operate in a manner analogous to modern-style plate tectonics.
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